Great efforts have been invested in the search for alternative fuels to reduce dependence on fossil fuels and to eliminate or reduce pollution associated with the burning of fossil fuels. Hydrogen is the most attractive alternative fuel because of its enormous heat of combustion (highest than that of any other material) and most environment friendly products (water vapor). In addition, hydrogen is the most abundant element in the universe.
Despite its widespread availability and obvious virtues, so-far hydrogen has not been utilized as a fuel of choice due to a number of technological problems which have not yet been solved satisfactory. These problems generally relate to devising safe, efficient, and economical methods of production, storage, transport, and utilization of hydrogen in sufficient quantities to make this fuel economically feasible.
Many methods have been described for the generation of hydrogen gas. The most common ones are electrolysis of water, gasification of coal, steam reforming of natural gas, partial oxidation of heavy oils, and the use of solar or nuclear reactor heat to break down steam into its component elements. However, these schemes for production of large quantities of hydrogen gas require major capital equipment, large production capabilities, and an input of significant amount of external energy.
Furthermore, neither of the above methods provides means for hydrogen storage, implying severe problems of storage, transportation, and safety. Hydrogen gas has extremely low density, and is highly explosive with air.
To overcome the problems of storage, handling, and transport of hydrogen, different methods have been proposed for generation of hydrogen on demand at the point of utilization:
Metal Hydrides for Storage and Production of Hydrogen:
certain metal-based compounds absorb hydrogen under moderate pressures (less than 7 MPa) at ambient temperatures, forming reversible metal-hydrogen compounds called hydrides. Metal hydrides can release hydrogen when the pressure is reduced and heat is applied. Although this method is considered a compact hydrogen storage (volume wise), practical hydrides contain relatively low mass fraction of hydrogen (in the commonly used iron titanium hydride the hydrogen mass fraction is only approximately 1.7%), implying low energy density. The production process of hydrides of relatively high hydrogen content is presently complex and very costly.
Reactions of Certain Metals with Water
(e.g., alkali metals, magnesium, aluminum) may be considered as a potential source of hydrogen and energy. Aluminum may serve as a very promising candidate for such a reaction because of the high amount of hydrogen (theoretically, 1.24 liter per gram of aluminum at standard conditions), high energy produced, easy and safe handling, availability, and relatively low cost. This reaction is also desirable because the main residue (aluminum hydroxide) is environmentally benign. Aluminum is not reactive in air at ambient conditions, whereas water is readily available and easy to store. In addition, one does not have to deal with hydrogen gas at any stage prior to its in-situ production for a direct use (e.g., for fuel cells, internal combustion engines, gas turbines, battery replacement, marine and underwater propulsion, etc.). Nevertheless, generally aluminum does not react with water (neither with air) in common ambient conditions due to a protective oxide (or hydroxide) layer naturally formed on the metal surface. In common applications, this “passivation” phenomenon is a fortunate property preventing corrosion, as long as the environment is not too acidic or alkaline. At the same time, it practically blocks the aluminum-water reaction. Hence, the execution of the reaction between aluminum and water to generate hydrogen requires activation, supposedly causing continuous disruption of the protective layer.
A number of approaches to increase reactivity of aluminum with water are known in the art:
One approach is a mechanical treatment (cutting or friction) of the aluminum to form fresh metal surfaces which can react with water.
For example, friction of metallic material under water and mechanical fracture of the oxide film accompanying the friction is disclose in U.S. Pat. No. 7,008,609.
K. Uehara, H. Takeshita and H. Kotaka in “Hydrogen gas generation in the wet cutting of aluminum and its alloys” (Journal of Materials Processing Technology, Volume 127, 2002, Pages 174-177) studied bubbling due to hydrogen generation during reaction of fresh aluminum surfaces with water.
Activation of the reaction of fine aluminum particles with water obtained by milling aluminum in water comprising a thermal shock treatment, where aluminum fine particles were repeatedly heated and cooled down, was described in U.S. Pat. No. 7,235,226.
Continuous removal of the passivation layer on aluminum by mechanical means, in order to sustain aluminum assisted water split reaction, has been disclosed in FR Pat. No. 2,465,683.
U.S. Pat. Nos. 5,052,272; 5,143,047; 5,712,442; and 5,789,696 describe controlled hydrogen generators that employ aluminum and water, where disruption of the protective oxide film is achieved by fast electrical heating of the metal above its melting point.
European patent No. 0 055 134 A1 discloses a method for the production of hydrogen by inducing electrical discharge between aluminum wire and aluminum drum both of which are immersed in water. When voltage is applied between the wire and drum, arching discharge takes place, helping disruption of the oxide layer formed on the wire tip, exposing fresh aluminum to the water. Thus, a continuous generation of hydrogen gas is possible when the wire is fed against the drum. This process has two major disadvantages. First, since the process is slow, the heat generated from the reaction is wasted by being dissipated through the water. Second, it is an energy demanding process: about 10 kJ of electrical energy per 1 gram of aluminum is required to sustain the reaction.
A second approach comprises the addition of alkali hydroxide, mainly sodium hydroxide or potassium hydroxide, to the water as disclosed in U.S. Pat. Nos. 2,721,789; 6,506,360; 6,638,493; 6,800,258; 6,834,623; 7,029,778; and US Patent Application 20040081615.
A similar approach comprises the presence of an effective amount of a catalyst in mixture with metal, wherein the catalyst is a water-soluble inorganic salt as disclosed in U.S. Pat. Nos. 3,932,600; 3,985,865; 6,440,385; 6,582,676; and US Patent Application 20050232837.
U.S. Pat. No. 6,582,676 and European patent application EU0417279A1 present the production of hydrogen from a water split reaction using aluminum and a ceramic, particularly calcium/magnesium oxide. Once contacted with water, these compounds cause substantial increase of pH (i.e. create alkaline environment), which stimulates corrosion of Al with accompanying release of hydrogen.
Unfortunately hydroxide chemicals cause very high alkalinity of the resulting products, making them corrosive, dangerous to handle, and potentially polluting to the environment. This increases the cost of the technology and adds safety and pollution problems. A further disadvantage is that the reaction products are not easy to handle and recycle.
Another approach comprises the use of alloys of aluminum with different metals for the reaction with water to produce hydrogen, as discloses in U.S. Pat. Nos. 4,182,748; 4,207,095; 4,324,777; 4,358,291; 4,752,463; 5,867,978; and 6,969,417.
The alloys comprise an alkali metal, mercury, and aluminum combined with a catalytically effective amount of metals comprising one or some of: platinum, palladium, germanium, antimony, gallium, tin, etc. The alloy is obtained by melting of the composition in an inert atmosphere. Sometimes, aluminum constitutes only small part of the alloy mass. Because of the production of the reactive alloy by melting, it is difficult to obtain the reactive material in a powdered form which would be advantageous for fast reaction with water due to the large surface area.
Additionally, amalgamation with, or the use of metals such as mercury; platinum; palladium; gallium; etc. results in an increase in the cost of the hydrogen to be produced. Furthermore, the use of mercury may be particularly objectionable in view of its toxicity. Additionally, the use of considerable quantities of alkali metals is disadvantageous from a process-technology point of view.
To summarize, most of the available methods for hydrogen production from the reaction between aluminum and water pose certain severe disadvantages such as complexity, high cost, toxicity, low yield, slow process, large amounts of an activating agent, etc.